Defective leptin production by adipose tissue is responsible for obesity in the ob/ob mouse. Plasma leptin concentration was measured in normal subjects by RIA (Linco, St. Louis, MO) using full-length recombinant human leptin as a standard. Repeated measurements in 15 subjects taken 7-10 days apart had a CV of 20%. Pasting leptin concentrations in 98 lean and obese normal subjects, 18-75 yrs old, were log-normally distributed (mean+SD=16.7+16.1 ng ml; range= 1.3297.9 ng/ ml). Plasma leptin levels correlated with percent body fat derived from skinfold thicknesses (r=0.771, P<0.0001), BMI (r=0.506, P<0.0001), plasma C-peptide (r=0.442, P<0.001), plasma insulin (r=0.239, P=.018), and percent of calories derived from carbohydrate (r=-0.239, P=0.019), but not with age, total calorie intake, or waist/hip ratio. Mean plasma leptin in women was 21.5 ng/ml compared to 6.5 ng/ml in men (P<0.0002 after adjusting for difference in body fat). In another group of 42 women, 60-70 yrs old, plasma leptin concentratins correlated with percent body fat derived from underwater weighing (r=0.807, P<0.0001), BMI (0.561, P<0.0001), plasma insulin (r=0408, P=0.007), C-peptide (r=0.426, P=0.006), HDL2 cholesterol (r=-0.384, P=0.01), apo Al (r=-0.33I, P=0.03), maximum oxygen uptake (r=-0.611, P<0.0001), and percent of calories derived from alcohol (r=-0.473, P<0.0001). Leptin concentration declined by 25.6+24% (P=0.0004) in 5 obese subjects (BMI=47.3) after only 10 days of dieting (1000 kcal/d). We conclude that plasma leptin concentration measured by RIA is reproducible in the same subjects over time. However, there is considerable inter-subject variation that is principally accounted for by body fat and, independently, by gender. Plasma leptin concentration correlates strongly with percent body fat but may also be regulated by other factors, such as plasma insulin, plasma lipoproteins, recent dietary intake, and aerobic fitness.